Detection and reduction of iron impurities in silicon solar cells

This paper presents a summary of some of the important properties of iron in silicon for solar cells. The importance of the chemical state of the Fe, and its impact on recombination activity in both pand n-type silicon, are discussed. Methods for the sensitive detection of dissolved Fe are also reviewed, including recent developments in high spatial resolution Fe imaging based on photoluminescence. Finally, some relevant aspects of gettering of Fe by phosphorus, boron and aluminum highly doped regions are discussed. Properties of iron in silicon Iron is one of the most common transition metal impurities in wafer-based silicon solar cells. It is especially important in multicrystalline silicon wafers, in which it occurs primarily due to contamination from the crucible during ingot growth [1,2]. Typical concentrations of Fe in directionally-solidified multicrystalline ingots are 10 10 cm in the central regions, and up to 10 cm near the bottom and top of the ingots [3,4], due to impurity segregation and solid-state in-diffusion of Fe from the crucible walls. Comparisons of the total Fe concentrations and dissolved Fe concentrations [4] have shown that the vast majority of the Fe (>95%) is usually present in precipitated form. These precipitates predominantly occur as iron silicide particles located at grain boundaries and dislocation clusters, and less frequently as iron silicate inclusions which may be present within the grains [1,5]. The remaining few per cent is present as dissolved Fe in the interstitial form. These interstitial Fe atoms are mobile even at room temperature, and courtesy of their positive charge state in p-type silicon, bond with negatively charged ionized dopant atoms in p-type silicon, usually boron, thus forming FeB pairs, although FeGa, FeIn and FeAl pairs can also occur [6]. The chemical state of Fe in silicon plays a key role in determining its recombination activity, which is of primary importance for solar cell performance. The recombination activity of precipitated Fe is difficult to specify, due to the fact that precipitates can vary greatly in size, may be present in a surrounding lattice with different degrees of imperfection, stress or strain, and also present a distribution of energy states in the band-gap, rather than a single energy level. However, the energy levels and capture cross sections of the recombination centers associated with interstitial Fe and its acceptor pairs have been fairly well documented. A selection of these are summarized in Table 1 from Refs [6-9]. We consider these values to be the most accurate, although there are widely varying values for the capture cross sections in the literature, and further refinement may occur. The values for FeGa and FeIn are less certain than those for Fei and FeB. Note that under illumination, Fe-acceptor pairs are dissociated, and so only interstitial Fe occurs in a working solar cell. However, the Fe-acceptor pairs lead to useful meta-stable properties that can be exploited for characterization purposes, as described below. A fundamental feature of isolated (i.e. un-paired) interstitial Fe in silicon is that it only presents a single donor level in the band-gap. This means that it may only have either a positive or neutral charge state, leading in turn to a relatively large capture cross section for electrons, and a relatively small capture cross section for holes The 6th International Symposium on Advanced Science and Technology of Silicon Materials (JSPS Si Symposium), Nov. 19-23, 2012, Kona, Hawaii, USA